A conventional interference canceller will now be explained. As the conventional interference canceller, there is known, for example, “a multi-user receiver” explained in Japanese Patent Application Laid-open No. 2000-138605.
Operations of the conventional interference canceller in 2000-138605 will now be explained. FIG. 15 is block diagram of the conventional interference canceller, i.e., the multi-user receiver. The conventional interference canceller removes known multi-user time-space interferences from high-rate user signals. In addition, the conventional interference canceller removes interferences under antenna directivity control using signals (interference removal residual signals) obtained by removing high-rate user signals from received signals.
In a code division multiple access (CDMA) system that multiplexes user signals at a plurality of transmission rates, the number of high-rate user signals is small; however, the influence of the signals in terms of interference is great. Conversely, while the CDMA system is less influenced by low-rate user signals, the number of the low-rate user signals is large and a hardware scale is made large. Therefore, the conventional interference canceller omits a multi-user interference canceller operation for each low-rate user signal.
This conventional interference canceller receives CDMA signals at antennas 101-1 to 101-N (where N is a natural number) and outputs high-rate user signals to high-rate user IEUs (Interference Estimation Unit; hereinafter, simply “IEU”) 103-1-1 to 103-1-K.
If the interference canceller consists of, for example, M stages of interference removal processing sections 102-1 to 102-M (where M is an integer equal to or greater than 2), IEU 103-1-1 to 103-M-K at each stage corresponding to the respective high-rate user signals receive interference removal residual signals for the respective antennas that are obtained by an interference removal processing at a previous stage and symbol replicas corresponding to the same user signals as those at the previous stage, and perform demodulation according to antenna directivities specific to the respective user signals. The IEU 103-1-1 to 103-M-K generate symbol replicas for the present stage and output the generated symbol replicas to the next stage.
At the same time, the IEU 103-1-1 to 103-M-K generate and output spread signals related to differences between the symbol replicas at the present stage and those at the previous stage for the respective antennas.
Delay units 105-1-1 to 105-(M-1)-N delay the received signals or interference processing residual signals for the respective antennas until processing results of the IEUs are output, respectively. Subtracters 106-1-1 to 106-1-(M-1)-N subtract outputs of the IEUs for the respective user signals from outputs of the delay units for the respective antennas, and obtain interference removal residual signals at the present stage for the respective antennas. The respective IEUs at a final stage output demodulated signals corresponding to the high-rate user signals.
Low-rate user DEMs (Demodulator Units: hereinafter, simply “DEM”) 104-1 to 104-K receive the interference removal residual signals that are obtained by the interference removal processing at an (M-1) stage for the respective antennas, demodulate the received signals according to the antenna directivities specific to the user signals, and output demodulated signals corresponding to the low-rate user signals, respectively.
FIG. 16 is a block diagram of the IEU. The IEU performs a path-based processing corresponding to a multipath channel having a plurality of paths (#1 to #P).
Despreading sections 111-1 to 111-N receive the interference removal residual signals of the previous stage, and despread the received signals for the respective antennas. Multipliers 112-1 to 112-N give weights W1 to WN to outputs of the respective despreading sections 111-1 to 111-N. An adder 113 synthesizes outputs of the multipliers 112-1 to 112-N. A multiplier 114 weights each symbol replica corresponding to the same user signal at the previous stage. An adder 115 adds up an output of the adder 113 and an output of the multiplier 114.
A detector 116 demodulates an output of the adder 115 using a channel estimate on each path. The detector 116 not only performs synchronized detection demodulation but also weighting for realizing maximum-ratio synthesis.
An adder 120 synthesizes outputs of the detector 116 for the respective paths. A decision unit 121 determines an output of the adder 120.
A multiplier 122 multiplies an output of the decision unit 121 by the channel estimate at each path to generate a symbol replica at the present stage, and outputs the generated symbol replica to the next stage. A subtracter 123 subtracts an output of the multiplier 114 from an output of the multiplier 122. A multiplier 124 weights an output of the subtracter 123. Multipliers 125-1 to 125-N multiply an output of the multiplier 124 by complex conjugate weights W1*/N to WN*/N that are obtained by normalizing the weights W1 to WN by the number of antennas, respectively. Spreading sections 126-1 to 126-N spread outputs of the multipliers 125-1 to 125-N for the respective antennas.
Adders 127-1 to 127-N add up outputs of respective paths of the spreading sections 126-1 to 126-N for the respective antennas. The IEU 103-1-1 to 103-1-K at the initial stage receive, as the interference removal residual signals at the previous stage, the signals received at the antennas and employ 0 as the symbol replicas corresponding to the same users as those at the previous stage. Each of the IEU 103-M-1 to 103-M-K at the final stage outputs only the demodulated signal that is an output of the adders 120 and does not perform the following interference estimation processing and interference removal residual signal update processing. As the weights W1 to WN, steering antenna weights or adaptive control weights decided based on estimates of user signal arrival directions, respectively are used. Further, weighting factors of the multipliers 114 and 124 are, for example, 1-(1-α)m-1 (where α is a real number equal to or smaller than 1 and m is the number of stages and an integer equal to or greater than 2 and equal to or smaller than M) and α, respectively.
FIG. 17 is a block diagram of the DEM. The DEM performs a path-based processing corresponding to the multipath channel having a plurality of paths.
Despreading sections 131-1 to 131-N receive the interference removal residual signals obtained by the interference removal processings at the previous stage (M-1) for the respective antennas and despread the received interference removal residual signals for the respective antennas. Multipliers 132-1 to 132-N weight outputs of the respective despreading sections. An adder 133 synthesizes outputs of the respective multipliers 132-1 to 132-N. A detector 134 demodulates an output of the adder 133 using the path-based channel estimate. An adder 135 synthesizes outputs of the detector 134 for the respective paths and output a demodulated signal.
As explained above, the conventional interference canceller removes the interferences from user signals having high signal power using the antenna directivity control and the multi-user interference canceller. The conventional interference canceller removes the interferences from user signals having low signal power based only on the antenna directivity control, and thereby attains a large interference removal effect with the canceller or receiver that is relatively small in scale.
However, the conventional interference canceller has the following disadvantage. Since it takes long time to converge beam formation because of an algorithm of an adaptive array antenna, the conventional interference canceller cannot deal with reception of a signal having a short time length such as a packet signal or a random access channel (hereinafter, “RACH”) signal.
The conventional interference canceller also has the following disadvantage. As the number of stages (the number of times of replica signal subtraction) of the interference canceller increases, signal-to-interference ratio (hereinafter, “SIR”) of the received signals improves. However, the demodulation processings up to the final stage are disadvantageously, greatly delayed. To be specific, this delay makes it impossible to deal with high-rate transmit power control (hereinafter, “TPC”) and characteristics of the canceller are disadvantageously, greatly deteriorated, depending on a change rate of fading.
It is an object of the present invention to solve at least the problems in the conventional technology.